Update changelong and upgrade

CHANGES.rst

@@ -5,6 +5,8 @@ compatible.
 
 The Matrix specification has been moved to a seperate git repository.
 
+You will also need an updated syutil and config. See UPGRADES.rst.
+
 Homeserver:
  * Sign federation transactions.
  * Rename timestamp keys in PDUs.

UPGRADE.rst

+Upgrading to v0.4.0
+===================
+
+This release needs an updated syutil version. Run::
+
+    python setup.py develop
+
+You will also need to upgrade your configuration as the signing key format has
+changed. Run::
+
+    python -m synapse.app.homeserver --config-path <CONFIG_> --generate-config
+
+
 Upgrading to v0.3.0
 ===================